The invention relates to a web-forming section of a paper or board machine which section is intended for manufacturing a multi-layer web and which comprises a first web-forming unit in which a first fiber layer is formed on a first wire, a second web-forming unit in which a second fiber layer is formed on a second wire, and a couch shoe over a curved deck of which the fiber layers are guided for joining them together.
The invention also relates to a method for manufacturing a multi-layer paper or board web.
When manufacturing a multi-layer web, separately formed fiber layers are usually joined together by means of a couch roll which is typically a perforated roll. An upper wire carrying the second fiber layer is guided over a sector of the couch roll which touches the route of the first fiber layer on a lower wire. After the couch roll, the web is detached from the upper wire and led forward carried by the lower wire.
A critical characteristic of multi-layer boards is the interlaminar strength of layers. Splitting or delamination is a problem which occurs especially with board machines running liquid-packaging grades and multi-layer grades. The question is then of coming loose of the upper or lower surface of the middle layer of a finished product, which causes severe problems in the end use of the product.
An important factor from the viewpoint of interlaminar strength is the amount of fines in the boundary of the fiber layers being joined. When one or both fiber layers are manufactured by gap former technique, low fines content in the boundaries being joined together can cause problems. Two-sided dewatering is known to generate fines washout from the vicinity of web surfaces. It can be generalized that the drier the fiber layers are when being joined together, the less there are fines in the boundaries and the worse they are bound together.
For improving inter-binding of layers, sometimes starch has been sprayed on the surface of separately formed fiber layers before joining them together. This can, however, cause problems in process management and cleansing.
Disadvantages related to the use of a couch roll are a sharp pressure pulse created in the nip and shear forces that cause orientation in the web. Furthermore, the rotation of the roll creates an underpressure pulse to the exit side of the nip which pulse can damage the web and, in the worst case, make the newly joined fiber layers separate from each other.
When using a couch roll, the solids content of the fiber layers being joined together has to be in a limited range which usually is 8-12%. At a too high solids content, it is difficult to obtain adequate interlaminar strength. A too low solids content can cause splashings and faults in the joined web. The solids content of fiber layers can be affected by dewatering equipment of web-forming units and their parameters. If one wishes to increase the production of a board machine, the alternatives are to add more effective dewatering equipment to the forming section and/or to increase the length of the wire section. Many times the extension of the wire section is not possible or cost-effective because of lack of space. In practice, the increase of capacity in a Fourdrinier machine means optimizing dewatering equipment and/or extension of the wire section. If both have already been done, increasing production is difficult.
The fiber layer coming from the upper wire unit is usually kept on the surface of the couch roll solely by adhesion forces between the fiber layer and the wire. At high running speeds, there is a risk that the fiber layer is detached from the wire in consequence of centrifugal force. U.S. Pat. No. 6,592,715 discloses an arrangement in which the couch roll or a couch shoe replacing it has been provided with a suction zone which is followed by an overpressure zone. The function of the suction zone is to make the second fiber layer adhere on the curved surface of the couch roll or shoe. In the overpressure zone, air or steam is blown to the web for detaching it from the upper wire. The fiber layers converge at a relatively sharp angle on the curved surface of the couch roll or shoe. Then, a sharp pressure pulse is created in the joint area which can damage the web being formed. The overpressure zone prevailing at the point of the nip further sharpens the pressure pulse being created.
From U.S. Pat. No. 4,425,187 is known a method for manufacturing a multi-layer paper web in which method a first fiber layer is formed on a first wire, and a second fiber layer is formed on a second wire. The fiber layers are led over a curved cover of a couch shoe and, simultaneously, between the fiber layers is fed a new stock layer from the headbox, whereby the couch gap also functions as a forming gap. The new stock layer brings more moisture and fines between the first and the second fiber layer, which improves the interlaminar strength of the layers of the web. The method and the apparatus used in it are, however, complex. Furthermore, dewatering from a so-called middle layer being fed between the fiber layers is difficult, because it takes place through two already drained fiber layers.
The object of the invention is to solve problems related to prior art.